The process which employs an alkaline solution containing ammonium for etchant purposes is of the type to which the regeneration or reconstituting of the present invention is to be applied.
An ammonia-based etchant has the advantage that it does not attack tin/lead or tin coatings which are commonly used to protect the copper circuit pattern. The most important advantage from a production standpoint is that an ammonia-based etch allows a high copper concentration in the etching solution and a large operating "window" due to its rather stable performance under a wide range of copper concentrations. The ammonia content provides the complexing agent which precludes copper precipitation at the operating pH; the inorganic cupric Cu.sup.++ ion is the actual oxidant dissolving the metallic copper. Various alkali etching systems have been used employing nitrates, chlorites and chlorides of copper in an ammonium complex. The solution regeneration procedure of the present invention has been developed and found to be economically applicable to all these formulations. However, modification is restricted to substituting an applicable anion used in the etch solution if it is not based on the chloride.
To provide a better understanding of the innovative features of our process dealing with regeneration and our reconstituting of a used etching solution, a brief outline of the physical-chemical reactions in such a system are set forth.
Cupric ion oxidizes the metallic copper of the coating in the reaction: EQU Cu.degree.+Cu.sup.++ .fwdarw.2Cu.sup.+ 1.
The cuprous ion formed is oxidized by air during a spraying action: EQU 2Cu.sup.+ +O.sub.2 .fwdarw.2Cu.sup.++ 2.
As an ammoniacal complex, this reaction remains basically unchanged: EQU Cu.degree.+Cu(NH.sub.3).sub.4.sup.++ +2Cl.sup.- .fwdarw.2Cu(NH.sub.3).sub.4.sup.+ +Cl.sup.- 3. EQU 2Cu(NH.sub.3).sub.2.sup.+ +2Cl.sup.- +O.sub.2 +4NH.sub.3 .fwdarw.2Cu(NH.sub.3).sub.4.sup.++ +2Cl.sup.- 4.
As indicated from reactions 3. and 4. above, the etching solution gains copper ions and consumes free chloride and ammonia. These changes as carried out in a production operation create a change in the solution composition. By adding ammonium chloride as a maintenance chemical, the solution balance can be re-established, but the continued copper gain adversely affects the etch rate at a certain point and, in a conveyorized proceess line, it adversely affects the quality of the product. Under pressure operating procedure, this problem is alleviated by a continuous, slow wastage of the etch solution. In larger production installations, an automatic density controller measures the copper gain that affects the specific gravity of the solution; a predetermined volume of etchant is then wasted and an equal volume of replenishing solution made up of ammonium chloride and free ammonia is added.
An ideal overall process will involve removing the copper solute from the etching solution at the same rate as it is gained from the etching operation. However, none of the heretofor available processes can accomplish this result in an economical and trouble-free manner. And, as a result, the spent etching solution is usually considered a waste and is sold at a fraction of the value of its copper content. A typical quotation is $0.05/gal. for a spent solution containing around 180 g/l of copper but the copper contained in that gallon, on the other hand, has a commercial value of about $1.10. Deductible from this meager rate of return are shipping and handling costs which vary in relation to the distance to the nearest recovery depot. A further and significant completely unrecovered loss in this type of utilization, is the ammonium chloride content which has an appreciable value independently of the copper which is lost, of $0.87/gal., based on a typical current vendor price for ammonium chloride solution. This is also a significant value factor.
The rinse waters also carry a significant quantity of copper and chemicals lost by dragout with the circuit boards which are about two to three times the normal dragout losses encountered in metal finishing, in view of the high density of the solution (sp. gr. 1.25). Obviously recovery from diluted rinse water is uneconomical when the processing solution, itself, has to be wasted due to the lack of a recovery method. It is estimated that this loss is approximately 6 gallons/500 sq. ft. of a one-sided circuit board surface that is being processed. A successful process in this connection will reduce the complexity of waste treatment problems the industry faces. Both copper and ammonia, in view of their concentrations, are considered toxic for aquatic biota and, if discharged into a sewer system, are toxic for bacteria in the sewerage treatment process and for surface waters into which treated sewage is discharged.
Thus, it has been an object of the present invention to develop what may be termed a closed loop or continuous regenerating system to reduce the complexity and the costs of waste treatment for a particular plant, whether it represents a relatively small or a large distillation.